Re: Flags in Ada?

[tmoran@bix.com]

>  type Permission_Type is
>     (Owner_Read, Owner_Write, Owner_Execute,
>      Group_Read, Group_Write, Group_Execute,
>      World_Read, World_Write, World_Execute);
>
>     -- These are the "bit names"
>
>  type Permission_Set_Type is array (Permission) of Boolean;
>  pragma Pack (Permission_Set_Type);
   -- and a rep clause to say just where these 9 bits fit? ;)

or

  type Line_Statuses is record
    Data_Ready   :boolean;
    Over_Run     :boolean;
    Parity_Error :boolean;
    Framing_Error:boolean;
    Break        :boolean;
    Transmitter_Holding_Register_Empty:boolean;
    Transmitter_Shift_Register_Empty  :boolean;
  end record;
  for Line_Statuses use record
    Data_Ready    at 0 range  0 .. 0;
    Over_Run      at 0 range  1 .. 1;
    Parity_Error  at 0 range  2 .. 2;
    Framing_Error at 0 range  3 .. 3;
    Break         at 0 range  4 .. 4;
    Transmitter_Holding_Register_Empty at 0 range 5 .. 5;
    Transmitter_Shift_Register_Empty   at 0 range 6 .. 6;
  end record;
  for Line_Statuses'size use 8;

In the Permissions example all the flags are homogenous - all 'OK or
DENIED' of some permission.  So
  Current_Permissions := (Permission_Type => False);
is a reasonable thing to say.  But in the UART example they have
quite different meanings (Data_Ready is a register status, Break
a line status, Parity_Error a data status) and normal values
(Parity_Error is much less often True than Data_Ready).
And though you might well wish to say
  Running_Status.Parity_Error
    :=Running_Status.Parity_Error or Latest_Status.Parity_Error;
it's unlikely you would want
  Running_Status.Data_Ready
    := Running_Status.Data_Ready or Latest_Status.Data_Ready;
so you would not say
  Running_Status:=Running_Status or Latest_Status;
You might of course mix the two styles - put Over_Run, Parity_Error,
and Framing_Error into a three component packed boolean vector a la
Permissions because they are similar to one another, and include
that bit vector along with Data_Ready, Break, and the Transmitter
statuses in a record a la Line_Statuses;